Walking, breathing, and eye movement are examples of essential everyday physiological activities that are powered by contractile activity of skeletal muscle. Skeletal muscles are inherently resting and contractile activity exclusively occurs in response to commands from the central nervous system. Such neuronal commands take the form of action potentials that travel from the brain to the muscle fibers in several steps. The neuromuscular junction (NMJ) is the highly specialized membrane area on muscle fibers where motor neurons come into close contact with the muscle fibers, and it is at NMJ that neurofnal action potentials are transmitted to muscular action potentials in a one-to-one fashion via synaptic transmission.
Neuromuscular transmission refers to the sequence of cellular events at the NMJ whereby an action potential in the lower motor neuron is transmitted to a corresponding action potential in a muscle fiber. When a neuronal action potential arrives at the pre-synaptic terminal it triggers influx of Ca2+ through voltage gated P/Q-type Ca2+ channels in the nerve terminal membrane. This influx causes a rise in cytosolic Ca2+ in the nerve terminal that triggers exocytosis of acetylcholine (ACh). Released ACh next diffuses across the synaptic cleft to activate nicotinic ACh receptors in the post-synaptic, muscle fiber membrane. Upon activation, ACh receptors convey an excitatory current flow of Na+ into the muscle fiber, which results in a local depolarization of the muscle fiber at the NMJ that is known as the endplate potential (EPP). If the EPP is sufficiently large, voltage gated Na+ channels in the muscle fiber will activate and an action potential in the muscle fiber will ensue. This action potential then propagates from NMJ throughout the muscle fiber and triggers the Ca2+ release from the sarcoplasmic reticulum. The released Ca2+ activates the contractile proteins within the muscle fibers thus resulting in contraction of the fiber.
Failure in the neuromuscular transmission can arise from both pre-synaptic dysfunction (Lambert Eaton syndrome, amyotrophic lateral sclerosis, spinal muscular atrophy) and as a result of post-synaptic dysfunction as occurs in myasthenia gravis. Failure to excite and/or propagate action potentials in muscle can also arise from reduced muscle excitability such as in critical illness myopathy (CIM). In Lambert Eaton syndrome, an autoimmune attack against the pre-synaptic P/Q-type Ca2+ channels results in markedly reduced Ca2+ influx into the nerve terminal during the pre-synaptic action potential and, consequently, a reduced release of ACh into the synaptic cleft. In myasthenia gravis the most common finding is an autoimmune attack on the post-synaptic membrane either against the nicotinic ACh receptors or the musk-receptor in the muscle fiber membrane. Congenital forms of myasthenia are also known. Common to disorders with neuromuscular transmission failure (Lambert Eaton syndrome, amyotrophic lateral sclerosis, spinal muscular atrophy and myasthenia gravis) is that the current flow generated by ACh receptor activation is markedly reduced, and EPPs therefore become insufficient to trigger muscle fiber action potentials. Neuromuscular blocking agents also reduce EPP by antagonizing ACh receptors. In CIM with reduced muscle excitability, the EPP may be of normal amplitude but they are still insufficient to trigger muscle fiber action potentials because the membrane potential threshold for action potential excitation has become more depolarized because of loss-of-function of voltage gated Na+ channels in the muscle fibers.
While ACh release (Lambert Eaton, amyotrophic lateral sclerosis, spinal muscular atrophy), ACh receptor function (myasthenia gravis, neuromuscular blockade) and function of voltage gated Na+ channels (CIM) are essential components in the synaptic transmission at NMJ, the magnitude of the EPP is also affected by inhibitory currents flowing in the NMJ region of muscle fibers. These currents tend to outbalance excitatory current through ACh receptors and, expectedly, they thereby tend to reduce EPP amplitude. The most important ion channel for carrying such inhibitory membrane currents in muscle fibers is the muscle-specific CIC-1 Cl− ion channel.
ACh esterase (AChE) inhibitors are traditionally used in the treatment of myasthenia gravis. This treatment leads to improvement in most patients but it is associated with side effects, some of which are serious. Because ACh is an import neurotransmitter in the autonomic nervous system, delaying it's breakdown can lead to gastric discomfort, diarrhea, salivation and muscle cramping. Overdosing is a serious concern as it can lead to muscle paralysis and respiratory failure, a situation commonly referred to as cholinergic crisis. Despite the serious side effects of AChE inhibitors, these drugs are today the treatment of choice for a number of disorders involving neuromuscular impairment. In patients where pyridostigmine (a parasympathomimetic and a reversible ACHE inhibitor) is insufficient, corticosteroid treatment (prednisone) and immunosuppressive treatment (azathioprine) is used. Plasma exchange can be used to obtain a fast but transient improvement.
Unfortunately, all of the currently employed myasthenia gravis drug regimens are associated with deleterious long-term consequences. In addition, the otherwise safe use of common drugs such as anti-infectives, cardiovascular drugs, anticholinergics, anticonvulsants, antirheumatics and others have been reported to worsen the symptoms of myasthenia gravis patients.
The CIC-1 ion channel is emerging as a target for potential drugs, although its potential has been largely unrealized.